Current investigations have established that patients with metastatic disease have increased consumption of platelets, fibrinogen, and plasminogen as measured by kinetic techniques. It has also been noted that appropriate treatment to reduce the tumor load also decreases the amount of consumption and conversely worsened disease increases consumption. This question is particularly important in children with acute leukemia where the activity of their disease produces marked consumption compromised by decreased platelet production due to tumor invasion. Thrombocytopenia is difficult to control with platelet transfusions because of the marked increase in the rate of destruction. The effect of donor platelets may be further reduced because of concurrent antibody formation or infection. Whether there is a difference in various states of leukemia in terms of amount of consumption and whether this can be correlated with parameters of extensive disease process are being investigated. In addition, since viral illnesses frequently occur in childhood and previous studies have suggested that thrombocytopenia is commonly associated, the effects of viral on platelet production and survival will be determined.